Electron emitter, charger, and charging method

ABSTRACT

Provided are an electron emitter continuously emitting electrons stably even in the atmosphere, a charger using the electron emitter, and a charging method using the charger. The electron emitter includes a electron emitting element consisting of a first electrode, a second electrode, and a semiconductor layer formed therebetween, and a power supply for alternately applying a positive voltage enabling electron emission and a negative voltage having a polarity opposite to the positive voltage. At least a part of the surface on the first electrode side of the semiconductor layer is formed of a porous semiconductor layer. Electrons captured in the porous semiconductor layer in the course of electron emission with application of a positive voltage disturb electron emission from the electron emitting element. Such electrons, however, are removed by application of a negative voltage.

TECHNICAL FIELD

The present invention relates to an electron emitter, a charger, and acharging method, and more particularly to an electron emitter, acharger, and a charging method for use in image forming devices such aselectrophotographic copy machines, printers or facsimiles.

BACKGROUND ART

Conventionally, in an image forming device such as anelectrophotographic copy machine, prior to forming an electrostaticlatent image at a body to be charged such as a photoreceptor, thesurface of the body to be charged is evenly charged by a variety ofmethods.

Conventional charging methods include, for example, corona discharge. Inthis method, the surface of a photoreceptor is charged by discharge froman extremely fine wire. This method, however, has a problem in that ahigh voltage power supply of about 4 to 10 kV is necessary to charge thesurface of a photoreceptor. Moreover, the discharge from the wire causesa large amount of ozone in the space between the wire and the surface ofthe photoreceptor, which adversely effects human bodies and alsoaccelerates degradation of the photoreceptor. In order to solve theproblem, a corona charger that is improved to decrease the resultingamount of ozone is disclosed, for example, in Japanese PatentLaying-Open Nos. 09-114192 and 06-324556.

Another charging method employs a contact charging scheme, which hasrecently been put into practical use. In this method, in order todecrease the resulting amount of ozone and the power consumption, aconductive member such as a conductive roller, brush, elastic blade orcarbon nanotube is brought into contact with a surface of aphotoreceptor to charge the surface of the photoreceptor.

Currently, a roller charging scheme using a conductive roller as aconductive member is widely utilized in light of the stability ofcharging. In the roller charging scheme, a conductive roller is abuttedunder pressure on a photoreceptor by receiving a voltage to attain thecharging of the photoreceptor. In the roller charging scheme, however,when the surface of the photoreceptor has any minute defect (pinhole),an abnormal amount of current leak occurs in the defect portion of thesurface of the photoreceptor from the conductive roller, causing thesurface of the photoreceptor to be destroyed, which may adversely affectimage formation.

As a further improvement to the roller charging scheme, for example,Japanese Patent Laying-Open No. 2001-296722 discloses a scheme in whicha secondary charging roller is added between a roller charging member(primary charging roller) and a photoreceptor. Here, the secondarycharging roller serves to carry charges from the primary charging rollerto the photoreceptor and aims to resolve the current leak problem causedby the pinhole in the photoreceptor. Also in this scheme, however, thecharging phenomenon is dominated by minute discharge created in thenarrow gap between the secondary charging roller and the photoreceptor.Therefore, it was impossible to completely remove ozone or NO_(x)produced during charging.

Furthermore, for example, Japanese Patent Laying-Open No. 2001-281964discloses that a carbon nanotube is applied to a contact-type charger.In the contact-type charger using a carbon nanotube, however, thepressing pressure of the carbon nanotube in contact with a photoreceptorcauses physical destruction of the carbon nanotube and accordinglyreduces the charging ability.

In addition, Japanese Patent Laying-Open No. 2001-331017 discloses acharger using an electron emitting element having an MIS(Metal-Insulator-Semiconductor) structure. In the electron emittingelement, a thin film electrode for forming an acceleration electricfield for electrons is provided on the front surface side of a poroussemiconductor layer, and an electrode for injecting electrons to theporous semiconductor layer is provided on the back surface side of theporous semiconductor layer. The electron emission principle andfabrication method for the porous semiconductor layer formed of a poroussilicon thin film is disclosed in detail in “Luminescence and RelatedNovel Functions of Quantum-sized Nanosilicon”, the Technical Report ofthe Institute of Electronics Information and Communication Engineer ofJapan, 1999-06, pp. 1-6. The charger using such an element only utilizeselectron attachment caused by electrons emitted from the electronemitting element in order to generate negative ions and therefore doesnot produce ozone or NO_(x) in principle as in the method usingdischarge as described above.

In the electron emitting element using the porous semiconductor layer,however, an electron, which attaches to a nanosized semiconductorparticle (nano-silicon crystal) constituting the porous semiconductorlayer due to charging (electron capture) caused during the operation ofelectron emission into the atmosphere, renders the electric field insidethe porous semiconductor uneven, thereby preventing acceleration ofelectrons. The amount of electron emission is thus reduced. The electronstored in the nanosized semiconductor particle by this charging exhibitsnonvolatility, and it is reported that some experimental result showsthat the electron attached to a nanosized semiconductor particle over aweek or longer. Generally, when this element is driven in theatmosphere, this charging causes electron emission from the electronemitting element to stop completely with continuous driving for aboutthree minutes.

DISCLOSURE OF THE INVENTION

In view of the circumstances as described above, an object of thepresent invention is to provide an electron emitter capable of beingstably driven for a long period of time, a charger using the same, and acharging method therefor.

The present invention provides an electron emitter including an electronemitting element having a semiconductor layer formed between a firstelectrode and a second electrode, in which at least a part of a surfaceof the semiconductor layer on a side of the first electrode is porous. Apower supply is provided for alternately applying a positive voltageenabling electron emission and a negative voltage having an oppositepolarity to the positive voltage to the first electrode.

Preferably, in the electron emitter of the present invention, anabsolute value of a magnitude of the negative voltage is at least 1.5times as large as an absolute value of a magnitude of an electronemission starting voltage of the electron emitter.

Preferably, in the electron emitter of the present invention, a ratiot1/t2 between an application time t1 of the positive voltage and anapplication time t2 of the negative voltage is at least 1 and at most1000.

Preferably, in the electron emitter of the present invention, aplurality of first electrodes are formed, and a power supply may beprovided for alternately applying respective voltages different inpolarity to at least one of the first electrodes and at least one of therest.

The present invention also provides a charger including theaforementioned electron emitter and a body to be charged arrangedopposing to and spaced apart from the first electrode of the electronemitter.

The present invention also provides a charging method in an electronemitter including an electron emitting element having a semiconductorlayer formed between a first electrode and a second electrode, in whichat least a part of a surface of the semiconductor layer on a side of thefirst electrode is porous. A positive voltage enabling electron emissionand a negative voltage having an opposite polarity to the positivevoltage are alternately applied to the first electrode of the electronemitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of a preferred exemplary chargerin accordance with the present invention.

FIG. 2 is a schematic structural view of an electron emitter and acounter electrode for use in an experiment in accordance with thepresent invention.

FIG. 3 is a diagram showing the relationship between an applied voltageand an electron emission current amount in the atmospheric pressure inthe electron emitter in accordance with the present invention.

FIG. 4 is a diagram showing a transition of an electron emission currentamount with respect to the elapsed time when a positive voltage iscontinuously applied to an acceleration electrode.

FIG. 5 is a diagram showing a transition of an electron emission currentamount with respect to the elapsed time when a positive voltage and anegative voltage are alternately applied to the acceleration electrode.

FIG. 6 is a diagram showing an exemplary waveform of a voltage appliedto the acceleration electrode of the electron emitter in accordance withthe present invention.

FIG. 7 is a diagram showing a transition of a diode current amount withrespect to an applied voltage to be applied to the accelerationelectrode of the electron emitter in accordance with the presentinvention.

FIG. 8 is a diagram showing another exemplary waveform of a voltageapplied to the acceleration electrode of the electron emitter inaccordance with the present invention.

FIG. 9 is a schematic perspective view of a part of an electron emitterin accordance with another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the embodiments of the present invention will bedescribed. It is noted that in the figure of the specification, the samereference characters refer to the same parts or corresponding parts.

FIG. 1 shows a schematic conceptual view of a preferred exemplarycharger in accordance with the present invention. This charger 1includes an electron emitter 10 and a photoreceptor 7 as a body to becharged that is arranged opposing to and spaced apart from an electronemitting surface 12 that is a surface of an acceleration electrode 6 ofelectron emitter 10.

Electron emitter 10 includes an electron emitting element 11 constitutedwith a base electrode 2 formed of a conductive substrate, an n-typesilicon layer 3 formed on base electrode 2, a non-doped thin polysiliconlayer 4 formed on n-type silicon layer 3, a porous polysilicon layer 5formed by rendering a part of polysilicon layer 4 porous, and anacceleration electrode 6 formed of a gold thin film formed on porouspolysilicon layer 5. Electron emitter 10 further includes a drivingpower supply 20 electrically connected to each of base electrode 2 andacceleration electrode 6 to enable supply of a voltage having a pulsewaveform, a sinusoidal waveform, a triangular waveform or the like.

On the other hand, photoreceptor 7 is formed on a surface of adrum-shaped conductive supporting substrate 8 made of aluminum or thelike with a thickness of approximately 25 μm. A bias power supply 21serving as a direct current voltage source is connected to conductivesupporting substrate 8.

Here, in the charger 1 as shown in FIG. 1, driving power supply 20 ofelectron emitter 10 applies a positive voltage to acceleration electrode6, and bias power supply 21 applies a positive voltage to conductivesupporting substrate 8. Then, the electron supplied from driving powersupply 20 to base electrode 2 is accelerated to the accelerationelectrode 6 side by the electric field inside electron emitting element11, which is produced by application of a positive voltage toacceleration electrode 6. Then, the accelerated electron is attracted toconductive supporting substrate 8 receiving a positive voltage, so thatthe electron is emitted from electron emitting surface 12, which is thesurface of acceleration electrode 6, and attaches to the surface ofphotoreceptor 7.

Here, in the present invention, driving power supply 20 alternatelyapplies to acceleration electrode 6 a positive voltage enabling electronemission and a negative voltage having a polarity opposite to thepositive voltage. Therefore, even if an electron is captured in anano-silicon crystal constituting porous polysilicon layer 5 in thecourse of electron emission, a negative voltage having a polarityopposite to a positive voltage is thereafter applied to accelerationelectrode 6, so that the captured electron can be removed from thenano-silicon crystal. Accordingly, when a positive voltage is applied toacceleration electrode 6 again, the electron accelerated to theacceleration electrode 6 side inside electron emitting element 11 isstably emitted outside electron emitting element 11 without beingaffected by the electron captured in the nano-silicon crystal in porouspolysilicon layer 5.

In this way, because of the alternate application of a positive voltageand a negative voltage to acceleration electrode 6, while the electronscaptured in the nano-silicon crystal constituting porous polysiliconlayer 5 are removed, electrons can be emitted outside electron emittingelement 11. Therefore, charger 1 and electron emitter 10 in accordancewith the present invention can be driven stably for a long period oftime.

Here, the absolute value of the magnitude of the negative voltage ispreferably 1.5 or more times as large as the absolute value of themagnitude of the electron emission starting voltage of electron emitter10. In this case, a sufficient amount of current flows in the directionfrom base electrode 2 to acceleration electrode 6 inside electronemitting element 11, so that the electron captured within porouspolysilicon layer 5 may tend to be removed from porous polysilicon layer5 effectively. It is noted that “electron emission starting voltage”refers to a voltage that is applied to acceleration electrode 6 whenelectron emission from electron emitter 10 is started.

A ratio t1/t2 between an application time t1 of a positive voltage andan application time t2 of a negative voltage is preferably 1 or more and1000 or less. When t1/t2 is 1 or more, electron emission from electronemitter 11 may tend to be performed sufficiently. In particular, when t1is sufficiently longer than t2, electron emission may tend to beperformed continuously since the period of time during which electronemission halts can be regarded as approximately zero. On the other hand,when t1/t2 is 1000 or less, an electron is not captured in porouspolysilicon layer 5 and stable electron emission tends to be enabled.Furthermore, with t1 set within 3 seconds, even more stable electronemission is enabled.

The forgoing was found by the present inventor according to theexperimental result as described below.

First, FIG. 3 shows the relationship between a voltage (V) applied toacceleration electrode 6 and an amount of electron emission current(A/cm²) produced by the electron emitted from electron emitter 10 shownin FIG. 2 to flow into a counter electrode 9, in the atmosphericpressure (room atmosphere). In FIG. 3, the axis of abscissas shows avalue of a voltage applied to acceleration electrode 6 for 1 second andthe axis of ordinates shows the mean value of the amount of the electronemission current flowing per 1 cm² of counter electrode 9 for 1 secondby the application of the voltage. Here, the distance between electronemitting surface 12 of acceleration electrode 6 and the surface ofcounter electrode 9 as shown in FIG. 2 is 1 mm and the bias voltageapplied to counter electrode 9 by bias power supply 21 is +100V.

In electron emitter 10 shown in FIG. 2, the electron emission currentstarts to be measured when the applied voltage shown in FIG. 3 becomes+8V. Thereafter, the amount of electron emission current increases asthe applied voltage rises from +8V.

In the atmospheric pressure, however, when a positive voltage iscontinuously applied to acceleration electrode 6 of the electron emitter10, as shown in FIG. 4, the amount of electron emission current (A/cm²)decreases exponentially. This is because electrons are graduallycaptured in the nano-silicon crystal constituting porous polysiliconlayer 5. It is noted that in FIG. 4 the axis of abscissas shows theelapsed time (minute) immediately after +18V is applied to accelerationelectrode 6.

Now, FIG. 5 shows the result of the measurement of the amount ofelectron emission current in counter electrode 9 when a positive voltageand a negative voltage are alternately applied to acceleration electrode6 of electron emitter 10. Here, the waveform of the voltage applied toacceleration electrode 6 is in the form of pulses as shown in FIG. 6. InFIG. 6, the application time of a positive voltage enabling electronemission is t1, and the application time of a negative voltage is t2. Inaddition, the ratio between t1 and t2 (t1/t2) is 2.

As shown in FIG. 5, when a negative voltage is applied to accelerationelectrode 6 at certain intervals, the amount of electron emissioncurrent (A/cm²) is kept stable as compared with the case shown in FIG.4.

It is noted that the value of the positive voltage applied toacceleration electrode 6 is preferably decided according to the amountof electron emission current as required. Here, it is set at +18V. Thevalue of the negative voltage is decided according to the followingexperimental result.

FIG. 7 shows the result of the measurement of the amount of diodecurrent flowing in electron emitting element 11 when voltages areapplied to acceleration electrode 6 of electron emitter 10 shown in FIG.2 for 1 second at intervals of 2 V in the order of 0V→−18V→0V→+18V→0V.In FIG. 7, the axis of abscissas shows a voltage (V) applied toacceleration electrode 6 and the axis of ordinates shows the amount ofdiode current (A/cm²) flowing per 1 cm² of electron emitting element 11.It is noted that in FIG. 7 a positive value of the amount of diodecurrent shows that the diode current flows in the direction fromacceleration electrode 6 to base electrode 2 (forward direction), and anegative value of the amount of diode current shows that the diodecurrent flows in the direction from base electrode 2 to accelerationelectrode 6 (reverse direction).

As described above, the electron emission starting voltage of electronemitting element 11 is +8V. In the course of 0V→−18V, the diode currentin the reverse direction starts to flow after the applied voltage is setto −10V, and the current amount is −300 μA/cm² when the applied voltageis set to −12 V.

As shown in FIG. 4, when a positive voltage is continuously applied toacceleration electrode 6, electrons are captured in porous polysiliconlayer 5 and the amount of electron emission current gradually decreases.The captured electron is held for an extremely long period of time whenthe diode current is continuously fed in the forward direction or whenacceleration electrode 6 is left in an open state (it is reported thatthe electron was held for one week or longer according to “LightEmission and Novel Function of Quantum Size Nano-Silicon,” the TechnicalReport of the Institute of Electronics Information and CommunicationEngineer of Japan, 1999-06, pp. 1-6). However, the captured electron canbe removed from porous polysilicon layer 5 by applying a negativevoltage to acceleration electrode 6 to cause an appropriate amount ofdiode current to flow in the reverse direction. Here, when a negativevoltage of −12 V or higher is applied to acceleration electrode 6, thecaptured electrons are completely removed from porous polysilicon layer5. Accordingly, the amount of electron emission current can be restoredto the approximately initial value.

Therefore, in order to restore the electrons captured in porouspolysilicon layer 5 to the initial state, the absolute value of themagnitude of the negative voltage (12V) applied to accelerationelectrode 6 should be 1.5 or more times as large as the absolute valueof the magnitude of the electron emission starting voltage (8V) ofelectron emitter 10.

The similar characteristics as described above are also obtained whenthe ratio t1/t2 between the positive voltage application time t1 and thenegative voltage application time t2 is varied. In other words, whent1/t2 is 1 or more and 1000 or less, the captured electrons can berestored to the initial state stably. However, if t1 is too long,depending on the design of porous polysilicon layer 5, the influence ofelectron capture appears in the amount of electron emission current.Thus, t1 is preferably set within 3 seconds at longest.

A preferred exemplary method of manufacturing electron emitter 10 havingsuch a structure in accordance with the present invention will bedescribed below. First, n-type silicon layer 3 is formed on baseelectrode 2. Then, non-doped polysilicon layer 4 having a film thicknessof about 1.5 μm is formed on the surface of n-type silicon layer 3, forexample, by CVD (Chemical Vapor Deposition) method. Thereafter,polysilicon layer 4 as an anode and a platinum electrode as a cathodeare soaked in a mixture solution of hydrogen fluoride aqueous solutionand ethanol. With polysilicon layer 4 being irradiated with light, aconstant current (30 mA/cm²) is fed between these electrodes for ananodic oxidation treatment. This anodic oxidation treatment renders apart of polysilicon layer 4 porous, resulting in porous polysiliconlayer 5. Thereafter, this stacked structure is removed from thesolution, and the surface of porous polysilicon layer 5 is subjected tothermal oxidation for 1 hour at about 900° C. with oxygen gas flow fedat a rate of 300 ml/min. Then, a thin film formed of gold is formed at athickness of about 10 nm on the surface of porous polysilicon layer 5 bya deposition method or a sputtering method to form accelerationelectrode 6. Electron emitting element 11 is thereby formed. Finally,driving power supply 20 is electrically connected to each of baseelectrode 2 and acceleration electrode 6, whereby electron emitter 10 inaccordance with the present invention is formed.

It is noted that although gold is used as a material of accelerationelectrode 6 in the foregoing description, aluminum or the like may beused.

Alternatively, in the present invention, the waveform of the voltageapplied to acceleration electrode 6 may be a sinusoidal waveform asshown in FIG. 8. In this case, the reference potential of the sinusoidalwave may not necessarily be 0 V. Furthermore, a direct current componentmay be superposed as long as such a condition is satisfied that theabsolute value of the magnitude of the negative voltage is 1.5 or moretimes as large as the absolute value of the magnitude of the electronemission starting voltage of electron emitting element 11. Specifically,the application of a voltage having a sinusoidal waveform of 1 Hz at apeak value of 18V enables even more stable electron emission.

FIG. 9 shows a schematic perspective view of a partial electron emitterin accordance with another embodiment of the present invention.

This electron emitter 10 is characterized by two accelerationelectrodes, that is, an acceleration electrode 6 a and an accelerationelectrode 6 b, which are not electrically connected to each other.Acceleration electrodes 6 a, 6 b are arranged respectively parallel tothe longitudinal direction of electron emitter 10. Here, using anot-shown power supply, when a positive voltage is applied toacceleration electrode 6 a, a negative voltage is applied toacceleration electrode 6 b, and when a positive voltage is applied toacceleration electrode 6 b, a negative voltage is applied toacceleration electrode 6 a. In other words, respective voltages havingdifferent polarities are alternately applied to acceleration electrode 6a and acceleration electrode 6 b.

In this manner, when a positive voltage is applied to accelerationelectrode 6 a, the electrons captured in porous polysilicon layer 5below acceleration electrode 6 b can be removed while electrons beingemitted from acceleration electrode 6 a. On other hand, when a negativevoltage is applied to acceleration electrode 6 a, electrons can beemitted from acceleration electrode 6 b while electrons captured inporous polysilicon layer 5 below acceleration electrode 6 a beingremoved. Since electrons can continuously be emitted by repeatedlyperforming these operations alternately, the surface of a body to becharged can bear electrons uniformly.

It is noted that, not being limited to two as described above, aplurality of acceleration electrodes may be arranged such as three orfour. When the number of acceleration electrodes is increased, thecharging distribution is effectively made uniform on the surface of thebody to be charged, and in addition electron emitter 10 can be drivenwith a margin. Therefore, the life of electron emitter 10 isadvantageously prolonged.

The charger 1 and electron emitter 10 in accordance with the presentinvention as described above is suitably used especially in imageforming devices such as electrophotographic copy machines, printers,facsimiles since it can be driven stably for a long period of time.

In accordance with the present invention as described above, there canbe provided an electron emitter that can be stably driven for a longperiod of time, a charger using the same, and a charging methodtherefor.

It should be understood that the embodiments disclosed herein should betaken by way of illustration rather than by way of limitation in allaspects. The scope of the present invention is shown not in theforegoing description but in the claims, and it is intended thatequivalents to the claims and all modifications within the claims areembraced herein.

INDUSTRIAL APPLICABILITY

The present invention is suitably used especially in image formingdevices such as electrophotographic copy machines, printers, orfacsimiles.

1. An electron emitter for emitting electrons into a surroundingatmosphere toward a counter electrode disposed in opposing, spaced apartrelation thereto, said electron emitter comprising an electron emittingelement having a semiconductor layer formed between a first electrodeand a second electrode, at least part of a surface of said semiconductorlayer on a side of said first electrode being porous, characterized inthat a power supply is provided for alternately applying a positivevoltage and a negative voltage having an opposite polarity to saidpositive voltage to said first electrode, said positive voltage being atleast sufficient to enable electron emission, wherein said electronemitter is further characterized in that an absolute value of a minimummagnitude of said negative voltage, which gives rise to a minimumnegative current flow in said electron emitter, is at least 1.5 times aslarge as an absolute value of a minimum magnitude of said positivevoltage, which gives rise to a minimum positive current flow in saidelectron emitter.
 2. The electron emitter according to claim 1,characterized in that a ratio t1/t2 between an application time t1 ofsaid positive voltage and an application time t2 of said negativevoltage is at least 1 and at most
 1000. 3. The electron emitteraccording to claim 1, characterized in that a plurality of said firstelectrodes are formed, and in that a power supply is provided foralternately applying voltages different in polarity to at least one ofsaid first electrodes and at least one of the rest.
 4. A chargercomprising an electron emitter according to claim 1 wherein said counterelectrode comprises a body to be charged that is arranged opposite toand spaced apart from a surface of said first electrode of said electronemitter.
 5. A charging method for charging a body to be chargedcomprising: providing an electron emitter for emitting electrons into asurrounding atmosphere toward said body to be charged disposed inopposing, spaced apart relation thereto, said electron emitter includingan electron emitting element having a semiconductor layer formed betweena first electrode and a second electrode, at least part of a surface ofsaid semiconductor layer on a side of said first electrode being porous,and alternately applying a positive voltage and a negative voltagehaving an opposite polarity to said positive voltage to said firstelectrode, said positive voltage being at least sufficient to enableelectron emission from said electron emitter, wherein said electronemitter is further characterized in that an absolute value of a minimummagnitude of said negative voltage, which gives rise to a minimumnegative current flow in said emitter, is at least 1.5 times as large asan absolute value of a minimum magnitude of said positive voltage, whichgives rise to a minimum positive current flow in said emitter.